Voltage source converters or other source inverters e.g. current or impedance source inverters find application in a wide range of technical fields. For example, they can be used as inverters, where one or more loads are driven by a DC voltage source, or they can be used as active rectifiers to generate a DC-bus from an AC voltage source. Power management circuitry between the source and the load(s) ensures efficient and safe distribution of the power to the load(s).
In the case of inverters, power from a DC source is provided to one or more loads, usually via a filter and then via a DC-link, including a DC-link capacitor, which smooths the supply voltage to the load(s). If the source is AC, a rectifier will also be provided to convert to DC before the DC-link.
Such source converters are in recent times finding greater application in the aircraft industry where there is a trend away from hydraulic systems and pneumatic systems to so-called ‘more electric aircraft’ (MEA) using electric subsystems, such as electrical actuators, and controls. In the case of actuators, source converters are applied as motor drives.
Particularly in aircraft applications, but also in other applications where such source converters are used, weight, size, cost and reliability are all important factors. The DC-link capacitor usually represents a significant portion of the size of the system. Therefore, it would be desirable to reduce the size of the DC link capacitor as much as possible.
With the objective of reducing size and weight, as well as cost and complexity, new electric actuation architectures are being proposed in which two or more motor drive circuits share the same DC-link. These motor drive circuits may operate different loads/electric machines.
One of the driving factors for DC-link capacitor size is the current ripple flowing through it. It has been found, however, that the motor drive controllers can be synchronised so as to reduce overall ripple at the DC-link. Then the DC-link components including the DC-link capacitor can be smaller. Filter requirements can also be reduced.
The impact of interleaving carriers on a DC-link in inverters has been studied for parallel voltage source converters, in Zhang, Wang, Burgos, Boroyevich, “DC-Link Ripple Current Reduction for Paralleled Three-Phase Voltage-Source Converters With Interleaving”, IEEE Trans. Power Electron., September 2011; and Open access PhD thesis from Virginia Tech available in the following link (see pages 46 to 71): https://theses.lib.vt.edu/theses/available/etd-05072010-115156/unrestricted/Zhang_Di_D_2010(Updated).pdf. Also, interleaving of parallel drives to minimise switching ripple on the AC side of such systems is described in U.S. Pat. No. 7,109,681.
It has been demonstrated in the prior art that interleaving carriers on paralleled voltage source inverters can effectively reduce the DC-link current ripple. This is illustrated in FIG. 2, where two inverters are shown. The dotted line and dashed line carriers would correspond to the modulation of each inverter, respectively.
There is, however, a need for an improved control for synchronising operation of a system having non-paralleled source inverters, or motor drives for respective, separated loads but sharing a common DC-link, whilst minimizing the required DC-link capacitance.